EPR Kinetic evidence for radical intermediacy in the oxidation of secondary amines to nitrones by [WO(O2)2OCOC5H4N]-[Bu4N+]

Oxidation reactions of N,N-benzylalkylamines by [WO(O2)2OCOC5H4N]-[Bu4N+] to nitrones were kineticallystudied by UV and EPR techniques. The reactions follow a second-order rate law and the rate-determiningstep is a simple bimolecular attack of amine onto the peroxide oxygen of the peroxometal complex, whichleads to the formation of the corresponding hydroxylamine. However, EPR measurements and iterativeprocedures point out that the reaction occurs through the intermediacy of aminoxyl radicals, formed by oxidationof hydroxylamine generated in situ in the rate-determining step, and subsequent oxidations of these radicalsto nitrones by the starting peroxo complex. It is suggested that the oxidation of hydroxylamine to aminoxylradical as well as the oxidation of aminoxyl radical to nitrone occurs through an electron transfer step associatedwith a proton transfer.